In the paper industry, corrugated board sheets are manufactured starting from a continuous corrugated board material, which is slit longitudinally and divided into strips. Each strip is further divided transversely to generate a plurality of sheets of desired length. Sheets thus obtained are delivered to a so-called stacker, which forms stacks or bundles of sheets. The stacks are subsequently delivered to the final user, for example for the manufacturing of corrugated board boxes or the like.
Corrugated board web is usually formed by a combination of at least one flat paper web and at least one fluted paper web. Fluted paper webs are usually obtained by corrugating a flat paper web between corrugator rollers meshing with each another. Usually, a cardboard web comprises at least one fluted paper web arranged between two flat paper webs, known also as “liners”. The liners are glued to the fluted paper web by means of glue applied to the crests of the flutes of the fluted paper web. Sometimes a cardboard web comprises more than just one fluted paper web. Intermediate liners are in this case arranged between two fluted paper webs. The flutes of the fluted paper webs may differ in terms of flute pitch and/or flute dimension. Different flutes are used to impart different mechanical properties to the final corrugated board sheet.
Fast advancing sheets must be carefully piled up to form stacks of regular shape. Known sheet stackers usually comprise a sheet conveyor arrangement which receives a substantially continuous flow of sheets which are shingled and delivered onto a stacking surface in a stacking bay.
In some cases, each stack is formed by staggered bundles, each bundle containing a predetermined number of sheets. TW-M423688U, US2014/0353119 and US2009/0169351 disclose sheet stackers configured and controlled for forming stacks of mutually staggered bundles of corrugated board sheets. In order to mutually stagger neighboring bundles of the stack, said stack is formed on a horizontally movable stacker platform. The reciprocating staggering motion is in a direction substantially parallel to the feeding direction of the corrugated board sheets. The stacker platform comprises a conveyor belt, forming a stacking surface. The conveyor belt has a horizontal conveying motion, orthogonal to the reciprocating staggering motion of the stacker platform. The conveyor belt is used to evacuate the formed stack from the stacking bay according to an evacuation direction which is substantially orthogonal to the direction of arrival of the corrugated board sheets in the stacking bay. Each bundle of a stack is formed against a single stop plate or a dual stop plate, which are arranged in two positions which are staggered along the direction of arrival of the corrugated board sheets. Staggering of neighboring bundles is obtained by means of a reciprocating motion of the stacker platform in a horizontal direction. Moving the entire stacker platform is difficult and requires strong actuators and a particularly sturdy structure.
CN204057396U and CN203255778U disclose further embodiments of stackers designed and configured for producing stacks of sheets, each formed by a plurality of staggered bundles. Staggering is obtained by using two mutually spaced apart stop plates. The distance between the stop plates is equal to the staggering of neighboring bundles. In addition to moving the stop plates, the sheet discharge end of the sheet conveyor must also be reciprocatingly moved back and forth in a direction parallel to the feeding direction, to achieve correct staggering of adjacent bundles.
In other known sheet stackers, smooth stacks are formed, as disclosed e.g. in U.S. Pat. No. 4,273,325.
U.S. Pat. No. 5,829,951 discloses an up-stacker, i.e. a sheet stacker wherein the stacks are formed on a stationary stacker platform, and wherein a sheet conveyor arrangement is provided having a downstream sheet discharge end, wherefrom the sheets are discharged onto the stack being formed, moves gradually upwards as the stack grows vertically. This known stacker is suitable for the formation of small stacks or bundles of sheets.
One of the critical aspects of sheet stackers is the transient phase of removing the formed stack from the stacking bay. Removing the stack requires a gap to be formed in the otherwise continuous flow of sheets delivered by the sheet conveyor arrangement to the stacking bay. The longer the time required for removing a just formed stack of sheets from the stacking bay, the larger the gap required in the sheet flow. This transient phase slows down the operation of the sheet stacker and thus adversely affects the production rate thereof. Also, forming a large gap in the sheet flow can be difficult.
Corrugated board sheets are produced according to jobs. Each job contains a certain number of identical cardboard sheets. A job may include a large number of sheets, e.g. several tens or even hundreds of sheets, which may form one or several identical stacks.
In some cases, however, smaller jobs must be processed. For instance small jobs containing just some tens of sheets are sometimes requested. Jobs may differ from one another with respect to the kind of liners and fluted paper webs used, as well as with respect to the dimensions of the sheets. While usually a stack includes identical sheets of the same job, in some circumstances it may be beneficial to collect different jobs on one and the same stack, in order to save space along the conveyors and in the storage areas. When different jobs are stacked in the same stack, each job is formed by a bundle of identical sheets. Stacked bundles may be formed by sheets of different length, such that one job may overhangingly project from the previous job or from the next job in the stack. This may prejudice the stability of the stack. The difference in length between sheets of jobs collected on the same stack cannot be larger than a given amount, to prevent the stack from collapsing. This poses limits on the possibility of stacking different jobs on the same stack.
A need therefore exists, to provide sheet stackers and methods which overcome or at least partly alleviate one or more of the drawbacks of known stackers and stacking methods.